Quantum dot white and colored light-emitting devices

ABSTRACT

A light-emitting device comprising a population of quantum dots (QDs) embedded in a host matrix and a primary light source which causes the QDs to emit secondary light and a method of making such a device. The size distribution of the QDs is chosen to allow light of a particular color to be emitted therefrom. The light emitted from the device may be of either a pure (monochromatic) color, or a mixed (polychromatic) color, and may consist solely of light emitted from the QDs themselves, or of a mixture of light emitted from the QDs and light emitted from the primary source. The QDs desirably are composed of an undoped semiconductor such as CdSe, and may optionally be overcoated to increase photoluminescence.

[0001] This application is a divisional of U.S. application Ser. No.09/350,956, filed Jul. 9, 1999, which claims benefit of U.S. applicationSer. No. 09/167,795, filed Oct. 7, 1998, which claims benefit of U.S.Provisional Application 60/092,120, filed Apr. 1, 1998, the disclosuresof which are incorporated herein by reference in their entirety.

[0002] This invention was made with government support under ContractNumber 94-00034 awarded by the National Science Foundation. Thegovernment has certain rights in the invention.

[0003] A portion of the disclosure of this patent document containsmaterial that is subject to copyright protection. The copyright ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent file or records, but otherwise reserves allcopyrights whatsoever.

FIELD OF THE INVENTION

[0004] The present invention relates to the use of quantum dots inlight-emitting devices. The invention further relates to light-emittingdevices that emit light of a tailored spectrum of frequencies. Inparticular, the invention relates to a light-emitting device, whereinthe device is a light-emitting diode.

BACKGROUND OF THE INVENTION

[0005] Light-emitting devices, in particular, light-emitting diodes(LEDs), are ubiquitous to modern display technology. More than 30billion chips are produced each year and new applications, such asautomobile lights and traffic signals, continue to grow. Conventionaldevices are made from inorganic compound semiconductors, typicallyAlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue).These devices emit monochromatic light of a frequency corresponding tothe band gap of the compound semiconductor used in the device. Thus,conventional LEDs cannot emit white light, or indeed, light of any“mixed” color, which is composed of a mixture of frequencies. Further,producing an LED even of a particular desired “pure” single-frequencycolor can be difficult, since excellent control of semiconductorchemistry is required.

[0006] Light-emitting devices of mixed colors, and particularly whiteLEDs, have many potential applications. Consumers would prefer whitelight in many displays currently having red or green light-emittingdevices. White light-emitting devices could be used as light sourceswith existing color filter technology to produce full color displays.Moreover, the use of white LEDs could lead to lower cost and simplerfabrication than red-green-blue LED technology.

[0007] White LEDs are currently made by combining a blue LED with ayellow phosphor to produce white light. However, color control is poorwith this technology, since the colors of the LED and the phosphorcannot be varied. This technology also cannot be used to produce lightof other mixed colors.

[0008] It has been proposed to manufacture white or coloredlight-emitting devices by combining various derivatives ofphotoluminescent polymers such as poly(phenylene vinylene) (PPVs). Onedevice that has been proposed involves a PPV coating over a blue GaNLED, where the light from the light-emitting device stimulates emissionin the characteristic color of the PPV, so that the observed light iscomposed of a mixture of the characteristic colors of the device and thePPV. However, the maximum theoretical quantum yield for PPV-baseddevices is 25%, and the color control is often poor, since organicmaterials tend to fluoresce in rather wide spectra. Furthermore, PPVsare rather difficult to manufacture reliably, since they are degraded bylight, oxygen, and water. Related approaches use blue GaN-based LEDscoated with a thin film of organic dyes, but efficiencies are low (see,for example, Guha et al. (1997) J. Appl. Phys. 82(8):4126-4128; III-VsReview 10(1):4, 1997).

[0009] It has also been proposed to produce light-emitting devices ofvarying colors by the use of quantum dots (QDs). Mattoussi et al. (1998)J. Appl. Phys. 83:7965-7974; Nakamura et al. (1998) Electronics Lett.34:2435-2436; Schlamp et al. (1997) J. Appl. Phys. 82:5837-5842; Colvinet al. (1994) Nature 370:354-357. Semiconductor nanocrystallites (i.e.,QDs) whose radii are smaller than the bulk exciton Bohr radiusconstitute a class of materials intermediate between molecular and bulkforms of matter. Quantum confinement of both the electron and hole inall three dimensions leads to an increase in the effective band gap ofthe material with decreasing crystallite size. Consequently, both theoptical absorption and emission of QDs shift to the blue (higherenergies) as the size of the QDs gets smaller. It has been found that aCdSe QD, for example, can emit light in any monochromatic color, inwhich the particular color characteristic of the light emitted isdependent only on the QD's size.

[0010] Currently available light-emitting diodes and related devicesthat incorporate quantum dots use QDs that have been grown epitaxiallyon a semiconductor layer. This fabrication technique is most suitablefor the production of infrared light-emitting devices, but devices inhigher-energy colors have not been achieved by this method. Further, theprocessing costs of epitaxial growth by currently available methods(molecular beam epitaxy and chemical vapor deposition) are quite high.Colloidal production of QDs is a much more inexpensive process, but QDsproduced by this method have generally been found to exhibit low quantumefficiencies, and thus have not previously been considered suitable forincorporation into light-emitting devices.

[0011] A few proposals have been made for embedding colloidally producedQDs in an electrically conductive layer in order to take advantage ofthe electroluminescence of these QDs for a light-emitting device.Mattoussi et al. (1998), supra; Nakamura et al. (1998), supra; Schlampet al. (1997), supra; Colvin et al. (1994), supra. However, such devicesrequire a transparent, electrically conductive host matrix, whichseverely limits the available materials for producing devices by thismethod. Available host matrix materials are often themselveslight-emitting, which may limit the achievable colors using this method.

SUMMARY OF THE INVENTION

[0012] In one aspect, this invention comprises a device, comprising alight source and a population of QDs disposed in a host matrix. The QDsare characterized by a band gap energy smaller than the energy of atleast a portion of the light from the light source. The matrix isdisposed in a configuration that allows light from the source to passtherethrough. When the QD disposed in the host matrix is irradiated bylight from the source, that light causes the QDs to photoluminescesecondary light. The color of the secondary light is a function of thesize, size distribution and composition of the QDs.

[0013] In one embodiment of this aspect, the QDs comprise a core of CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe,InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixturethereof, and are, optionally, overcoated with a shell materialcomprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs,GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs,AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. Preferably, theband gap energy of the overcoating is greater than that of the core. Thecore or core-shell QD may be further coated with a material having anaffinity for the host matrix. The host matrix may be any polymer, suchas polyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene,polyvinyl, poly-diacetylene, polyphenylene-vinylene, polypeptide,polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene,polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate,hydrogel, agarose, cellulose, and the like. The primary light source maybe a light-emitting diode, a laser, an arc lamp or a black-body lightsource. The color of the device is determined by the size, sizedistribution and composition of the QDs. The size distribution may be arandom, gradient, monomodal or multimodal and may exhibit one or morenarrow peaks. The QDs, for example, may be selected to have no more thana 10% rms deviation in the diameter of the QDs. The light may be of apure color, or a mixed color, including white.

[0014] In a related aspect, the invention comprises a method ofproducing a device as described above. In this method, a population ofQDs is provided, and these QDs are dispersed in a host matrix. A lightsource is then provided to illuminate the QDs, thereby causing them tophotoluminesce light of a color characteristic of their size, sizedistribution and composition. The QDs may be colloidally produced (i.e.,by precipitation and/or growth from solution), and may comprise a coreof CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe,HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixturethereof. The QDs are, optionally, overcoated with a shell materialcomprising ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaP, GaAs,GaSb, HgS, HgSe, HgTe, InAs, Inp, InSb, AlAs, AlP, AlSb, an alloythereof, or a mixture thereof. The host matrix may be any material inwhich QDs may be dispersed in a configuration in which they may beilluminated by the primary light source. Some examples of host matrixmaterials include polyacrylate, polystyrene, polyimide, polyacrylamide,polyethylene, polyvinyl, poly-diacetylene, polyphenylene-vinylene,polypeptide, polysaccharide, polysulfone, polypyrrole, polyimidazole,polythiophene, polyether, epoxies, silica glass, silica gel, siloxane,polyphosphate, hydrogel, agarose, cellulose, and the like. Any lightsource capable of causing the QDs to photoluminesce may be used; someexamples are light-emitting diodes, lasers, arc lamps and black-bodylight sources.

[0015] It may be desirable to tailor the size distribution of the QDs ofa particular core composition to tailor the color of light which isproduced by the device. In one embodiment, referred to herein as a“monodisperse size distribution,” the QDs exhibit no more than a 10% rmsdeviation in diameter. The light may be of a pure color using amonodisperse size distribution of QDs or of a mixed color using apolydisperse size distribution of QDs, including white.

[0016] In a further aspect, the invention comprises a QD composition, inwhich QDs are disposed in a host matrix. The QDs are, optionally, coatedwith a material having an affinity for the host matrix. When illuminatedby a source of light of a higher energy than the band gap energy of theQDs, the QDs photoluminesce in a color characteristic of their size,size distribution and composition.

[0017] In one embodiment, the QDs comprise a core of CdS, CdSe, CdTe,ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP,InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixture thereof, and are,optionally overcoated with a shell material comprising ZnO, ZnS, ZnSe,ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO,HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP, AlSb, an alloythereof, or a mixture thereof. The host matrix may be a polymer such aspolyacrylate, polystyrene, polyimide, polyacrylamide, polyethylene,polyvinyl, polydiacetylene, polyphenylene-vinylene, polypeptide,polysaccharide, polysulfone, polypyrrole, polyimidazole, polythiophene,polyether, epoxies, silica glass, silica gel, siloxane, polyphosphate,hydrogel, agarose, cellulose, and the like. In one embodiment, the QDsare coated with a monomer related to a polymer component of the hostmatrix. The QDs may be selected to have a size distribution exhibitingan rms deviation in diameter of less than 10%; this embodiment willcause the QDs to photoluminesce in a pure color.

[0018] A related aspect of the invention comprises a prepolymercomposition comprising a liquid or semisolid precursor material, with apopulation of QDs disposed therein. The composition is capable of beingreacted, for example by polymerization, to form a solid, transparent ortranslucent host matrix, i.e., a host matrix that allows light to passtherethrough. Optionally, the QDs are coated with a material having anaffinity for the precursor material or with a prepolymeric material. Forexample, if the prepolymer composition forms a polyacrylate uponpolymerization, the QD can be coated with an acrylate monomer which,optionally, allows the QD to become incorporated into the backbonestructure of the polymer. The precursor material may be a monomer, whichcan be reacted to form a polymer. The QDs may comprise a core of CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe,InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixturethereof, and are, optionally, overcoated with a shell materialcomprising ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs,GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs,AlN, AlP, AlSb, an alloy thereof, or a mixture thereof. The QDs may beselected to have a size distribution having an rms deviation in diameterof less than 10%.

[0019] In yet another aspect, the invention comprises a method ofproducing light of a selected color. The method comprises the steps ofproviding a population of QDs disposed in a host matrix, and irradiatingthe QDs in the host matrix with a source of light having an energyhigher than the band gap energy of a QD in the host matrix such that theQDs are caused to photoluminesce. The QDs may comprise a core of CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe,InAs, InP, InSb, AlAs, AlP, AlSb, an alloy thereof, or a mixturethereof, and are, optionally overcoated with shell material comprisingZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP,GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs, AlN, AlP,AlSb, an alloy thereof, or a mixture thereof. The host matrix maycomprise polymers such as polyacrylate, polystyrene, polyimide,polyacrylamide, polyethylene, polyvinyl, poly-diacetylene,polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone,polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silicaglass, silica gel, siloxane, polyphosphate, hydrogel, agarose,cellulose, and the like.

[0020] The host matrix containing the QDs may be formed by reacting aprecursor material having QDs disposed therein (for example bypolymerization or physically entrapping). Alternatively, two or moreprecursor materials may be provided, each having QDs of a differentsizes, size distributions and/or compositions disposed therein. Theseprecursors may be mixed and reacted to form a host matrix, oralternatively, they may be layered to form a host matrix havingdifferent sizes, size distributions and/or compositions of QDs indifferent layers.

BRIEF DESCRIPTION OF THE DRAWING

[0021] The file of this patent contains at least one drawing executed incolor. Copies of this patent with color drawings(s) will be provided bythe Patent and Trademark Office upon request and payment of thenecessary fee.

[0022] The invention is described with reference to the several figuresof the drawing, which are presented for the purpose of illustrationonly, and in which,

[0023]FIG. 1 represents one embodiment of a light-emitting deviceaccording to the invention;

[0024]FIG. 2 represents another embodiment of a light-emitting deviceaccording to the invention;

[0025]FIG. 3 represents yet another embodiment of a light-emittingdevice according to the invention; and

[0026]FIG. 4 is a color photograph of several suspensions of QDs inhexane, illustrating the wide range of colors that can be achieved bythe methods and devices of the invention.

DETAILED DESCRIPTION

[0027] The practice of the present invention will employ, unlessotherwise indicated, conventional methods of chemistry within the skillof the art. Such techniques are explained fully in the literature.

[0028] As used in this specification and the appended claims, thesingular forms “a,” “an” and “the” include plural references unless thecontent clearly dictates otherwise. Thus, for example, reference to “aquantum dot” includes a mixture of two or more such quantum dots, a“layer” includes more than one such layer, and the like.

[0029] In describing the present invention, the following terms will beemployed, and are intended to be defined as indicated below.

[0030] The term “quantum dot” or “QD” as used herein is intended toencompass a core nanocrystal, an overcoated core (“core-shell”)nanocrystal, a coated core-shell nanocrystal or a coated core, unlessthe context clearly indicates otherwise.

[0031] The phrase “colloidally grown” quantum dots is used herein torefer to QDs which have been produced by precipitation and/or growthfrom a solution. A distinction between these QDs and quantum dotsepitaxially grown on a substrate is that colloidally grown QDs have asubstantially uniform surface energy, while epitaxially grown QDsusually have different surface energies on the face in contact with thesubstrate and on the remainder of the QD surface.

[0032] As used herein, the terms “pure” or “monochromatic” color refersto a color which is composed of light of a narrow distribution ofwavelengths having a spectral width between about 10-100 nm, preferablybetween about 10-50 nm, and more preferably about 10-30 nm. A “mixed” or“polychromatic” color refers to a color which is composed of light of amixture of different monochromatic colors.

[0033] The term “monomer” is intended to refer to a substance that canbe polymerized according to techniques known in the art of materialsscience, and may include oligomers. A “related monomer” of a polymer isa component monomer of the polymer, or a compound capable of beingincorporated into the backbone of the polymer chain.

[0034] The term “affinity” is meant to describe the adherence between aQD with a coat material and a host matrix. The adherence may compriseany sort of bond including, but not limited to, covalent, ionic, orhydrogen bonding, Van der Waals' forces, or mechanical bonding, or thelike.

[0035] “Optional” or “optionally” means that the subsequently describedevent or circumstance may or may not occur, and that the descriptionincludes instances where the event or circumstance occurs and instancesin which it does not. For example, the phrase “optionally overcoatedwith a shell material” means that the overcoating referred to may or maynot be present in order to fall within the scope of the invention, andthat the description includes both presence and absence of suchovercoating.

[0036] Light-emitting devices of almost any color visible to the humaneye can be produced by the techniques of the current invention using asingle undoped semiconductor material for the QDs. Embodiments of theinvention are illustrated in FIGS. 1 and 2, and indicated generally at10 and 100, respectively. In general terms, the invention comprises aprimary light source 13, for example a light-emitting diode, a laser, anarc lamp or a black-body light source. The primary light source 13 isdesirably chosen so that its energy spectrum includes light of higherenergies than the desired device color energy emitted by the QDs. Theprimary light source is disposed so as to irradiate a host matrix 12containing a population of QDs 14. The primary light source is inoptical communication with the host matrix. In one embodiment, primarylight source 13 is in physical contact with the host matrix. Optionally,a medium 11 is interposed between host matrix 12 and primary lightsource 13. Medium 11 may be a medium transparent or translucent to orconductive of at least a portion of the light emitted from primary lightsource 13, e.g., air, a vacuum, a polymer, a glass, a liquid or thelike. The interposition of medium 11 between primary light source 13 andhost matrix 12 can result in the light source being physically separatefrom the host matrix.

[0037] Host matrix 12 may be any material in which QDs can be disposedand that is at least partially transparent or translucent to, i.e.,allows light to pass therethrough, or conductive of light from primarylight source 13; examples of suitable host matrices are discussedfurther below. The host matrix 12 desirably contains a dispersion of QDs14, wherein the size, size distribution and/or composition of the QDshas been selected to produce light of a given color. Otherconfigurations of QDs disposed in a host matrix, such as, for example, atwo-dimensional layer on a substrate with a polymer overcoating, arealso contemplated within the scope of the invention. Techniques forproducing QDs that fluoresce in a narrow spectral distribution of aselected color are discussed further below and in Dabbousi et al. (1997)J. Phys. Chem. B 101:9463-9475 and in copending U.S. patent applicationSer. No. 08/969,302, “Highly Luminescent Color Selective Materials,”Bawendi et al, filed Nov. 13, 1997; such techniques allow particularlyfine color control of the final light-emitting device. However, othertechniques for producing QDs and disposing them in a host matrix arealso encompassed within the scope of the invention.

[0038] The primary light source 13 and the size, size distribution andcomposition of the QDs 12 are chosen in such a way that the radiationemitted from the device is of the desired color. The invention may beconstructed with a density of QDs such that substantially all light fromthe primary source is absorbed by the QDs and the radiation emitted fromthe device is produced principally by photoluminescence of the QDs.Alternatively, the invention may be constructed with a lower density ofQDs such that the light emitted from the device is a mixture ofunabsorbed primary light and of secondary light produced byphotoluminescence of the QDs. A very wide range of both pure and mixedcolors can be produced by a device constructed according to theprinciples of the invention.

[0039] For example, CdSe QDs can be produced that emit colors visible tothe human eye, so that in combination with a source of higher energythan the highest energy of the desired color, these QDs can be tailoredto produce visible light of any spectral distribution. FIG. 4 showsseveral suspensions of CdSe QDs made according to the method of Dabbousiet al., supra, and U.S. application Ser. No. 08/969,302, supra, andillustrates the very wide range of colors which can be achieved usingthe photoluminescence of these materials. The maxima of thephotoluminescent peaks in these solutions are (from left to right) (a)470 nm, (b) 480 nm, (c) 520 nm, (d) 560 nm, (e) 594 nm, and (f) 620 nm.The solutions are being irradiated by an ultraviolet lamp emitting 356nm ultraviolet light.

[0040] QDs can also be produced that emit in the ultraviolet and infrared spectral ranges. Examples of ultraviolet- and infrared-emitting QDsare, e.g., CdS, ZnS and ZnSe, and InAs, CdTe and MgTe, respectively.Such UV and IR emitters can also be incorporated into the devicedisclosed and claimed herein.

[0041] It is usually desirable that the QDs be isolated from each otherwithin the host matrix, particularly when the device is intended to emitlight of a mixed color. For example, when two QDs of different sizes arein close contact, the larger QD, which has a lower characteristicemission energy, will tend to absorb a large fraction of the emissionsof the smaller QD, and the overall energy efficiency of the device willbe reduced, while the color will shift towards the red.

[0042] In one particular embodiment of the invention, a whitelight-emitting device is provided. Such a device may be produced bycombining a combination of sizes of photoluminescent QDs with a standardblue primary light source. Referring to FIG. 1, the device, generallyindicated at 10, comprises a blue light source 13, for example an LED ofthe AlGaInN type, to provide primary light. This light passes through alayer or layers comprising QDs that luminesce in a lower-energy rangethan the blue LED embedded in a polymeric matrix. In the embodimentshown in FIG. 1, the primary light first passes through a layer 16 ofQDs 18 of a material and size adapted to emit red secondary light. Theprimary light which has not been absorbed by the first layer and thesecondary light then pass through a second layer 20 of QDs 22 of amaterial and size adapted to emit green secondary light. Once the lighthas passed through this second layer, it will be composed of a mix ofunabsorbed blue primary light, green secondary light, and red secondarylight, and hence will appear white to the observer. The relativeamplitudes of the red, green, and blue components of the light can becontrolled by varying the thickness and QD densities of the red andgreen layers to produce a light-emitting device of a desired color.

[0043] In another preferred embodiment, the red-emitting QDs 22 andgreen-emitting QDs 18 can be mixed in a common matrix 12, as shown inFIG. 2. The color can be controlled by varying the relative densities ofthe different sizes and compositions of QDs and the thickness of thelayer.

[0044] In yet another preferred embodiment, layers of host matrixcontaining QDs can be formed in a concentric conformation, e.g., aspherical or cylindrical conformation, as illustrated in FIG. 3.Indicated generally at 200, the device comprises layers of host matrix202, in which are dispersed QDs 204, and primary light source 220. Innerlayer 210 is prepared, for example, by providing a precursor materialhaving disposed therein a QD 216 having a size, size distribution,composition, or combination thereof, selected to emit in a predeterminedspectral range. The precursor material is reacted, e.g., polymerized, toform host matrix 210 having QDs 216 dispersed therein. These steps arerepeated as often as desired with the same or different precursormaterial having disposed therein QDs of the same or different size, sizedistribution, composition or combination thereof to form layers of hostmatrix 208 and 206 having disposed therein QDs 214 and 212,respectively, surrounding host matrix 210. If desired, a the QDs may beomitted from any layer. Primary light source 220 is disposed to be inoptical communication with the layers of host matrix 202 so as toirradiate the QDs 204 disposed therein. In one embodiment, primary lightsource 220 is in physical contact with the host matrix. Optionally,medium 218, as described above, is interposed between the layers of hostmatrix 202 and primary light source 220. When the host matrix isconformed as a cylinder, the primary light source can be disposed toirradiate the QDs in the host matrix from the base or the side of thecylinder.

[0045] In still another embodiment, the primary light source may be alight source such as a laser or a UV light source. In this embodiment,the QD layer(s) may comprise QDs emitting in a spectral range rangingfrom infrared to violet. By controlling the size, size distribution andcomposition of the QDs, the spectral distribution of the resulting lightmay be controlled.

[0046] When it is desired to produce a light-emitting device that emitsa particular color, rather than a white light-emitting device, this alsomay be accomplished by the practice of the invention. Although theinvention is expected to be particularly useful for the manufacture of alight-emitting device that produces polychromatic light (mixed colors),which are difficult to produce by traditional methods, light-emittingdevices that produce monochromatic light (pure colors) may also beprepared by the practice of the invention. This may be desirable forpurposes of ease of manufacturing, since substantially the same set ofequipment is required to produce light-emitting devices of almost anyvisible color, whether pure or mixed.

[0047] The perception of color by the human eye is well understood, andformulae for mixing pure colors to produce any desired mixed color canbe found in a number of handbooks. The color of light produced by aparticular size and composition of QD may also be readily calculated ormeasured by methods which will be apparent to those skilled in the art.As an example of these measurement techniques, the band gaps for QDs ofCdSe of sizes ranging from 12 Å to 115 Å are given in Murray et al.(1993) J. Am. Chem. Soc. 115:8706. These techniques allow readycalculation of an appropriate size, size distribution and composition ofQDs and choice of primary light source to produce a light-emittingdevice of any desired color.

[0048] When a white light-emitting device, e.g., a white LED, isdesired, an appropriate mix of QD sizes may be used. A white light whichappears “clean” to the observer may be achieved, for example, bytailoring the spectral distribution to match a black body distribution,e.g., as would be produced by a resistive lamp.

[0049] When a colored device, such as a blue AlGaInN LED, is used as theprimary light source, the color of the light generated by that devicemay or may not be included in the final spectrum produced by the deviceaccording to the invention, depending on the density of the QDs and thepath length of the light. If a sufficiently high density of QDs isprovided, the QDs will absorb substantially all of the primary light,and only secondary light in the characteristic colors of the QDs will beobserved. If a lower density of QDs is provided, a significant quantityof primary light may be mixed with the secondary light emitted by theQDs.

[0050] The host matrix will typically be a solid or liquid materialwhich is at least sufficiently transparent or translucent so that lightemitted by the QDs can be detected and in which QDs can be dispersed.For example, the host matrix can be a polymer, an epoxy, a silica glass,a silica gel, or a solvent, but any suitable material may serve as thehost matrix. The host matrix can be any material that is at leastpartially transparent or translucent to or conductive of light from theprimary light source. An advantage of the present invention compared tolight-emitting devices based on electroluminescence of QDs, rather thanphotoluminescence, is that in the present invention the host matrix neednot be electrically conductive. Electroluminescent QD LEDs require atransparent, electrically conductive material to serve as the hostmatrix. Such materials are rare, compared to the very large number oftransparent or translucent materials available for use with the presentinvention that are not necessarily conductive. Suitable host matrixmaterials for the devices described herein include many inexpensive andcommonly available materials, such as polyacrylate, polystyrene,polyimide, polyacrylamide, polyethylene, polyvinyl, poly-diacetylene,polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone,polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silicaglass, silica gel, siloxane, polyphosphate, hydrogel, agarose,cellulose, and the like.

[0051] A further advantage of the present invention is the manufacturingflexibility afforded by the use of multiple populations of QDs toachieve both pure and mixed colors of light. “Stock” solutions ofdifferent sizes, size distributions and compositions of QDs suspended ina monomer or other precursor material can be maintained, and mixed invarying amounts to produce almost any desired color. For example, threesuspensions of CdSe QDs in a liquid monomer such as styrene could beproduced: a first suspension of QDs of approximately 5.5 nm diameter(which will luminesce in the red), a second suspension of QDs ofapproximately 4.0 nm diameter (which will luminesce in the green), and athird suspension of QDs of approximately 2.3 nm diameter (which willluminesce in the blue). These suspensions function as a kind of “lightpaint”; by varying the amounts of these three suspensions, andpolymerizing the resulting mixture, light-emitting devices of a verywide range of colors can be produced using the same manufacturingtechniques, varying only the starting materials.

[0052] Preferably, colloidally produced QDs are coated such that theycan be dispersed in the host matrix without flocculation. In the case ofdispersal in a polymeric host matrix, use of a related monomer with apendent moiety possessing affinity for the QD's surface has been foundto allow good mixing of QDs into a polymer matrix. Particular cases ofthis type of coating may be found in the Examples. In the case ofdispersal in a prepolymer host matrix, use of a related monomer with apendent moiety possessing affinity for the QD's surface has been foundto allow good mixing into a monomer solution for subsequentpolymerization to form the host matrix. Particular cases of this type ofcoating may be found in the Examples. In the case of dispersal into asilica glass or gel, any coating that will bind at one end to the QD,and the other end of which has an affinity for the matrix, may be used.The coating may be applied directly to the surface of the QD or as acoating to an overcoated QD.

[0053] A number of methods of producing QDs are known in the art. Anymethod of producing QDs that will fluoresce with a desired spectrum maybe used in the practice of the invention. Preferably, the methodsdescribed in Dabbousi et al., supra, and U.S. application Ser. No.08/969,302, supra, can be used to produce QDs useful in devices asdisclosed and claimed herein. Dabbousi et al., supra, discloses a methodthat can be used for overcoating QDs composed of CdS, CdSe, or CdTe withZnS, ZnSe, or mixtures thereof. Before overcoating, the QDs are preparedby a method described in Murray et al., supra, that yields asubstantially monodisperse size distribution. An overcoat of acontrolled thickness can then be applied by controlling the duration andtemperature of growth of the coating layer. The monodispersity of thecore QDs results in monochromatic emission. The overcoated QDs,optionally, have improved quantum efficiency and emit more light thanunovercoated QDs.

[0054] The above method can be used to prepare separate populations ofQDs, wherein each population exhibits a different characteristicphotoluminescence spectrum. By mixing populations so prepared, a devicethat fluoresces in any desired mixed color, including white, may beproduced. The overcoating on the QDs allows the device to produce morelight than would be possible using unovercoated QDs.

[0055] Below are examples of specific embodiments of the presentinvention. The examples are offered for illustrative purposes only, andare not intended to limit the scope of the present invention in any way.

[0056] Efforts have been made to ensure accuracy with respect to numbersused (e.g., amounts, temperatures, etc.), but some experimental errorand deviation should, of course, be allowed for.

EXAMPLE 1 ODs in Polystyrene

[0057] A green light-emitting device has been constructed according tothe principles of the invention described above. The QDs used toconstruct this device were composed of a CdSe core and a ZnSovercoating. The absorption and luminescence properties of the QDs wereprimarily determined by the size of the CdSe core. The ZnS shell actedto confine electrons and holes in the core and to electronically andchemically passivate the QD surface. Both the core and shell weresynthesized using wet chemistry techniques involving formation of CdSeor ZnS from precursors added to a hot organic liquid as described below.

[0058] CdSe Core Synthesis

[0059] 16 ml of trioctylphosphine (TOP), 4 ml of 1 M trioctylphosphineselenide (TOPSe) in TOP, and 0.2 ml dimethylcadmium were mixed in aninert atmosphere (nitrogen-filled glovebox). 30 g of trioctylphosphineoxide (TOPO) was dried under vacuum at 180° C. for 1 hour, and thenheated to 350° C. under nitrogen. The precursor solution was theninjected into the TOPO. The temperature immediately fell to about 260°C. and CdSe nanocrystals immediately formed. The absorption peak of thenanocrystals immediately after injection was found to be around 470 nm.The temperature was held at 250-260° C. for about 10-15 minutes,allowing the nanocrystals to grow. During this time, the absorption peakshifted from 470 nm to 490 nm. The temperature was then dropped to 80°C. and held with the solution under nitrogen. The heat was removed andabout 15 ml butanol was added to prevent solidification of the TOPO asit cooled to room temperature. This process produced 12×10⁻⁶ moles (12μmoles) of CdSe QDs.

[0060] The UV-Vis absorption spectrum of the CdSe nanocrystals showed afirst transition peak at 486 nm with a half-width half-max (HWHM)measured on the red side of the peak, of 14 nm. This absorption peakcorresponded to a nanocrystal radius of 13 Å. The actual sizedistribution can be determined experimentally via small angle x-rayscattering or TEM. The absorption spectrum gave a rough estimate of thesize distribution. The 14 nm HWHM suggested a size distribution with aHWHM of about 1 Å.

[0061] ZnS Shell Synthesis

[0062] The CdSe core solution (15 ml; 2.22 μmoles) was used to producethe overcoated QDs. The nanocrystals were precipitated out of thesolution by slowly adding 40-50 ml of methanol. The precipitate was thenredispersed in hexane and filtered with 0.2 micron filter paper. 40 g ofTOPO was dried as described above and then cooled to 80° C. Thenanocrystals in hexane were injected into the TOPO, and the hexane wasevaporated under vacuum for 2 hours. A ZnS precursor solution was thenprepared in an inert atmosphere by mixing 4 ml of TOP, 0.28 ml ofdiethylzinc, and 0.56 ml of bistrimethylsilyl sulfide (TMSi)₂S. Theamounts of precursor were chosen to produce a ZnS shell thickness ofabout 9 angstroms, which corresponds to 4 monolayers at 2.3angstroms/monolayer. The nanocrystal/TOPO solution was then heated to140° C., and the precursor solution was added over 4 minutes. Thetemperature was then reduced to 100° C. and held at that temperature forat least two hours. Heat was removed and butanol added to preventsolidification of the TOPO.

[0063] The UV-Vis absorption spectrum of the overcoated QDs showed thefirst transition peak at 504 nm with a HWHM measured on the red side ofthe peak of 20 nm. The photoluminescence peak was at 520 nm.

[0064] Dispersal of QDs in Polymer

[0065] ZnS-overcoated QDs were dispersed in poly(styrene) as follows.ZnS-overcoated QDs (0.44 μmoles CdSe QDs) in TOPO/butanol wereprecipitated and then dispersed in hexane as described above. Hexane wasevaporated under vacuum from an aliquot containing 0.09 μmoles QDs. TheQDs were redispersed in 0.1 ml of toluene. n-Functionalized,amine-terminated polystyrene (molecular weight=2600; 0.05 g) wasdissolved in 0.2 ml toluene. 0.05 ml of toluene solution containing QDs(0.04 μmoles CdSe QDs) and 0.05 ml functionalized polystyrene in toluene(about 0.01 g) were mixed together and sonicated for about 10 minutes. Asolution of 1 g polystyrene (molecular weight=45,000) in 1 ml of toluenewas prepared. 0.1 ml of this concentrated polystyrene solution (about0.05 g polystyrene) was added to the QD/functionalized-polystyrenesolution. The resulting solution was sonicated for 2 minutes tothoroughly mix the QDs and polystyrene.

[0066] Production of Diode

[0067] The blue diode used as a primary light source was GaN based andhad a luminescence peak at 450 nm. The glass cap was a shortened,thin-walled glass tube (OD=5 mm, ID=4.3 mm, length={fraction (3/16)}″).The glass cap was filled with the QD/polymer solution and allowed to dryunder flowing nitrogen for over two hours. More QD/polymer solutioncould be added and dried as needed, but only one filling and drying stepwas needed for this diode. When dried, the polymer left a void at thebase of the cap. The emitting portion of the blue diode was then placedin this void at the base of the cap. The polymer itself did not contactthe diode. Green light was produced as the blue light from the GaNcaused the QDs to luminesce at 520 nm. The 520 nm light gave the devicea green appearance.

EXAMPLE 2 ODs in an Epoxy Polymer Matrix

[0068] CdSe/ZnS QDs having a 14 Å core radius were prepared as describedin Example 1. 0.01 μmoles of QDs in TOPO solution were taken, and theQDs were precipitated and washed 2 times with methanol. The QDs werethen redispersed in 0.27 ml (2 mmoles) of a capping monomer,6-mercaptohexanol. In order to effectively disperse the QDs in thecapping monomer, the solutions were first sonicated for about 10 minutesand then stirred for 2 hours at 50-60° C.

[0069] The QD solution was then further reacted with epoxide monomers.0.56 ml (2 mmoles) of poly[(phenyl glycidylether)-co-formaldehyde](number average molecular weight=345) and 0.08 ml (0.8 mmoles) ofdiethyltriamine were added to the 6-mercaptohexanol solution. Theresulting mixture was thoroughly mixed and placed in a glass tube havingan outside diameter of 6 mm and a length of 50 mm. Air bubbles formedduring mixing were removed by sonicating for 10 minutes. The glass tubecontaining the monomer mixture was then heated to 70° C. in an oil bathfor 2 hours, forming a high molecular weight epoxy with the QDsdistributed therein. This formed composite could then be used asdescribed in Example 1 with a primary light source to make a green LED.

EXAMPLE 3 ODs in a Methacrylate Polymer Matrix

[0070] CdSe/ZnS QDs having core radii of 13, 15, 18, 21, 23, 29, and 34A were prepared as described in Example 1. Solutions of between0.01-0.05 μmoles of each diameter of QD in TOPO were precipitated andwashed with methanol 2 times. 50-100 μl (100-200 μmoles) oftrioctylphosphine, freshly removed from a nitrogen-atmosphere glove box,were then added to each QD precipitate. 650 μl of lauryl methacrylate(Sigma-Aldrich, 96%, 2.2 mmoles) was added to each QD-trioctylphosphinesolution and stirred for 2 minutes. Approximately 350 μl of1,6-hexanediol dimethacrylate (Polysciences, 98%, 1.2 mmoles) was addedto each lauryl methacrylate solution and stirred for another 2 minutesto form a monomer solution of each different diameter QD. 10-20 mg ofazobisisobutylonitrile (AlBN, 1% w/w) was then added to each monomersolution. The resulting mixtures were individually mixed thoroughly andplaced in glass tubes having an outer diameter of 6 mm, an innerdiameter of approximately 4.5 mm, and a length of 50 mm. In a separateexperiment, blue gallium nitride LED primary light sources (Nichia,NSPB300A, epoxy-polymer encapsulated) were dipped into each of themonomer solutions until the monomer solution completely covered thediode head.

[0071] Each of these two types of devices was then placed in an oven,preheated at 70° C., for approximately 2 hours. Care was taken to avoiddisturbing the monomer mixture during polymerization. After 2 hours, themonomer was completely polymerized, i.e., it was firm on contact and itwas resistant to deformation under applied pressure. For the polymerizedspecimens without the LEDs, the glass tubes were scored with a file andbroken to yield polymerized QD-composite plastic sticks that emittedblue, blue-green, green, yellow, orange, red, or deep red light under UVexcitation. The LED-containing specimens emitted in the same colors,with the exception of blue, under excitation by the blue LED.

[0072] Mixed colored and white emitters can be constructed by mixingdifferent monomer solutions having different core radii CdSe QDs inthem. Surprisingly, polymerization does not reduce the quantum yields ofthe QDs, so the final color emitted by these mixed QD-polymer compositesis of the same energy and intensity as the initial mixture of monomersolutions.

[0073] Other embodiments of the invention will be apparent to thoseskilled in the art from a consideration of the specification or practiceof the invention disclosed herein. It is intended that the specificationand example be considered as exemplary only, with the true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of producing a light-emitting device,comprising: (a) providing a population of quantum dots (QDs) and a hostmatrix, wherein each QD of the population comprises a core of anindependently selected size and composition, the population comprises aselected size distribution of QDs, and the QDs have a surface adapted toallow the QDs to be dispersed in the host matrix; (b) dispersing the QDsin the host matrix; and (c) illuminating the QDs disposed in the hostmatrix with a light source that emits primary light which causes the QDsto photoluminesce secondary light, thereby causing the light-emittingdevice to emit light comprising a mixture of primary and secondarylight.
 2. The method of claim 1, wherein step (a) comprises growing theQDs by precipitation from a solution.
 3. The method of claim 1, whereinthe core comprises a material independently selected from the groupconsisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, GaAs, GaP, GaAs, GaSb,HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, AlSb, alloys thereof, andmixtures thereof.
 4. The method of claim 1, wherein step (a) furthercomprises coating the core with an overcoat.
 5. The method of claim 4,wherein the core overcoat comprises a material independently selectedfrom the group consisting of ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe,MgS, MgSe, GaAs, GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN,InP, InSb, AlAs, AlN, AlP, AlSb, alloys thereof, and mixtures thereof.6. The method of claim 4, wherein the core overcoat is produced byprecipitation from a solution.
 7. The method of claim 1, wherein step(a) further comprises coating the QDs with a coat material having anaffinity for the host matrix.
 8. The method of claim 7, wherein the hostmatrix comprises a polymer and the coat material comprises a relatedmonomer.
 9. The method of claim 1, wherein the host matrix comprises amaterial selected from the group consisting of liquids, polymers,epoxies, silica glasses, silica gels, and combinations thereof.
 10. Themethod of claim 1, wherein the host matrix comprises a polymer selectedfrom the group consisting of polyacrylate, polystyrene, polyimide,polyacrylamide, polyethylene, polyvinyl, poly-diacetylene,polyphenylene-vinylene, polypeptide, polysaccharide, polysulfone,polypyrrole, polyimidazole, polythiophene, polyether, epoxies, silicaglass, silica gel, siloxane, polyphosphate, hydrogel, agarose, andcellulose.
 11. The method of claim 1, wherein the light source isselected from the group consisting of a light-emitting diode, a laser,an arc lamp and a black-body light source.
 12. The method of claim 1,wherein the population of QDs has a size distribution having less than a10% rms deviation in diameter of the core.
 13. A method of preparing adevice, comprising: (a) providing a prepolymer composition, comprising(i) a precursor material capable of being reacted to form a solid hostmatrix that allows light to pass therethrough, and (ii) a population ofquantum dots (QDs) disposed in the precursor material, wherein each QDof the population comprises a core of an independently selected size andcomposition, and the population comprises a selected size distributionof QDs, (b) reacting the precursor material to form the host matrix withQDs dispersed therein; and (c) providing a primary light source inoptical communication with the host matrix with QDs dispersed therein.